1. Field of the Invention
This invention relates to sensor systems for measuring the concentration of gases dissolved in a liquid. In particular this invention relates to a sensor system which comprises a method and apparatus for measuring the concentration of dissolved gases in insulating and cooling oils used in electrical transformers.
2. Introduction to the Invention
Electrical transformers are known to degrade and fail due to age, high loads, varying loads, and environmental conditions. In some instances such failures can be catastrophic. It is also known that oils used for cooling and insulation of the transformers, as well as other insulation materials, e.g. cellulosic insulation (paper), can break down under the electrical and thermal stresses. The break down of the oil and other materials yields a variety of products, including certain gases which are often referred to as "fault gases." Such fault gases may include hydrogen (H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), methane (CH.sub.4), acetylene (C.sub.2 H.sub.2), ethylene (C.sub.2 H.sub.4), and ethane (C.sub.2 H.sub.6), and other gases. The generation of fault gases can be accelerated by the presence of water which degrades the performance of the insulating fluid.
In many cases, the degradation and ultimate failure of a transformer may occur over a fairly long period of time. The detection of the fault gases can be useful in detecting degradation and/or failure of the transformer or its components. Thus, it has been found to be useful to periodically sample and analyze the transformer oil to identify the composition and, in some cases, the concentration of fault gases present in the oil, in order to thereby assess the health of the transformer.
However, the methods and equipment used to date are cumbersome and time consuming. Some methods require oil and/or gas samples be extracted at the transformer site and then taken to a laboratory for analysis. Some methods do not provide data on multiple gases of interest. Some systems which do provide data on multiple gases require that the various gases be separated prior to analysis. Most methods require that a carrier gas, such as nitrogen, be injected to facilitate the transportation of extracted gases though the sensing system. As a result of these limitations, the transformers are typically monitored on an infrequent basis, perhaps once or twice per year. Thus, the danger of catastrophic failure, including explosion, of very expensive transformers persists with the attendant loss of power distribution to critical processes such as chemical and semiconductor plants, or to businesses and consumers.
A common, known method of analyzing transformer oil for fault gas concentration involves obtaining a sample of the transformer oil, typically by extracting the oil with a syringe, and taking the oil to a laboratory for analysis by means such as gas chromatography.
It is common, for example, in using such methods to use vacuum extraction to extract the fault gases from the oil. As a result, heavy hydrocarbons can be extracted. Such heavy hydrocarbons can be separated from the more important lighter hydrocarbons via gas chromatography due to their longer retention times on columns. However, in order to use infra red (IR) sensing techniques it becomes necessary to remove these heavy hydrocarbons from the gas mixture. This separation can be accomplished by condensing the heavy hydrocarbons in a cold trap but this makes the process more expensive and may require the added maintenance of cooling systems and liquid disposal. While laboratory analysis is still the most common practice, there have been prior attempts to provide information in real-time using an on-line system. It should be noted that for such a device to be useful it must operate over a wide ambient temperature range (e.g. -20.degree. C. to +50.degree. C.), be able to handle varying oil temperatures (e.g. 0.degree. C. to +140.degree. C.), and provide reliable data for many months without requiring service. To date, no such instrument has been realized.
Therefore, it is desirable to provide a sensor system capable of monitoring and measuring the concentration of fault gases in a liquid such as a transformer oil on a continuous, real-time basis using passive extraction techniques. It is desirable that such a system be capable of measuring the concentration of each of the fault gases of interest, and in the presence of other fault gases. It is further desirable that such a system be capable of unattended operation, requiring only periodic maintenance. It is also desirable that such a system be capable of: being remotely located with the transformer being monitored; sensing and determining the concentrations of fault gases present in the transformer oil; and transmitting resultant data to a central location for further analysis and follow-up action.